How to tweak render when each render takes so long?
There are a number of ways to deal with this:


render a frame at low resolution and/or samples (i.e thumbnail). This
should reduce render time but still give you are. This can
effectively be done with the preview render in the 3d view.  
learn more about optimising cycles settings (e.g. tile size, bounces, clamp)
render out to a multilayer EXR and use the compositor to perform final
tweaks rather than re-rendering. 
for animation, Opengl playblasts
(the clapperboard icon in the 3d view) allow you a quick preview of
how your animation will look, but without full materials and
lighting. This can be very useful when normal playback is not at full   
bake background props (inlcuding lighting etc) to textures once you
are satisfied with them, that way you don't need to re-render them
again (see http://www.creativeshrimp.com/light_baking_tutorial.html for a discussion of some of these techniques).